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Section: 21.2 [container.requirements] Status: NAD Submitter: Paolo Carlini Opened: 2007-11-11 Last modified: 2017-07-18
View all other issues in [container.requirements].
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In an emplace member function the function parameter pack may be bound to a priori unlimited number of objects: some or all of them can be elements of the container itself. Apparently, in order to conform to the blanket statement 21.2 [container.requirements]/11, the implementation must check all of them for that possibility. A possible solution can involve extending the exception in 21.2 [container.requirements]/12 also to the emplace member. As a side note, the push_back and push_front member functions are luckily not affected by this problem, can be efficiently implemented anyway.
[ Related to 767 and to 2164 ]
[ Bellevue: ]
The proposed addition (13) is partially redundant with the existing paragraph 12. Why was the qualifier "rvalues" added to paragraph 12? Why does it not cover subelements and pointers?
Resolution: Alan Talbot to rework language, then set state to Review.
[ 2009-07 Frankfurt ]
The problem is broader than emplace. The LWG doesn't feel that it knows how to write wording that prohibits all of the problematic use cases at this time.
LWG believes that 2164 addresses this issue and therefore considers 760 as NAD.
Add after 21.2 [container.requirements]/12:
-12- Objects passed to member functions of a container as rvalue references shall not be elements of that container. No diagnostic required.